Process for the preparation of hydrophilic membranes and such membranes

ABSTRACT

This invention relates to a process for the preparation of hydrophilic membranes by coagulation of a solution of at least one hydrophobic polymer and at least one hydrophilic polymer in a suitable solvent medium in a coagulation medium. The process according to the invention is characterized in that hydrophilic microfiltration and/or ultrafiltration membranes are prepared by cross-linking the hydrophilic polymer which is present in or on the obtained membrane matrix after the coagulation in an essentially non-swollen state and fixing same as such in or on the polymer matrix. Advantageously the solvents and the undesirable components of the coagulation medium are removed whereby the hydrophilic polymer is converted into an essentially non-swollen state prior to cross-linking. The removal of the undesirable components from the membrane takes place by means of a non-solvent for the hydrophilic polymer according to the invention. Finally, the invention relates to hydrophilic microfiltration- and/or ultrafiltration membranes, flat or tubular of a hollow fiber structure.

The invention relates to a process the preparation of hydrophilicmembranes by coagulation of a solution of a hydrophobic and hydrophilicpolymer in a suitable solvent medium in a coagulation medium, as well asto such hydrophilic membranes.

Such a process and membranes are known from the GermanOffenlegungsschrift DE No. 31 49 976.

The membranes, obtained according to this known process have a waterabsorption ability of at least 11%, by weight, based on the total weightof the membrane at a 100% relative humidity and 25° C.

Such membranes, which are asymmetrical, are especially suitable fordiffusive separations and because of their great water absorptionability.

However, the known membranes are not suitable for microfiltration and/orultrafiltration, since the pores of the known membranes are filled withswollen hydrophilic polymer that hinders a quick passage of a permeate.

The invention aims to provide a process in which the disadvantages areobviated in an effective way.

To this end the invention comprises a process for the preparation ofhydrophilic membranes by coagulation of a solution of a hydrophobicpolymer and a hydrophilic polymer in a suitable solvent medium in acoagulation medium, characterized in that hydrophilic microfiltrationand/or ultrafiltration membranes are prepared by cross-linking thehydrophilic polymer which is present in or on the obtained membranematrix after the coagulation in an essentially non-swollen state andfixing same as such in or on the polymer matrix.

The present invention results in hydrophilic membranes which areespecially suitable as microfiltration and/or ultrafiltration membranes.They may advantageously be applied where water is used due to their goodmoistening ability. The membranes according to the invention mayadvantageously be used for e.g. the separation of proteins, because theabsorption of same is reduced to a minimum which means that themembranes are nonfouling.

In this way the hydrophilic membranes prepared according to the presentprocess may be used by filtration of milk and dairy products, filtrationof foodstuff, sterile filtration of liquids, plasmaferese andbloodfiltration and for the preparation of ultra pure water.

It will be clear that the application of the hydrophilic membranesobtained according to the invention is not restricted to the abovementioned application.

Further, the hydrophilic membranes according to the invention have asimportant properties that they posses a good mechanical sturdiness, areresistant to a great number of chemicals and moreover will stand hightemperatures. Such membranes are in growing demand since they may beapplied for the above mentioned separation purposes in an economicallyjustified manner, the more so since the hydrophilic membranes accordingto the invention have a water permeability up to 8000 l/m² /hr/bar as aresult of a good porosity and the hydrophilic character of the membranematerial.

A favourable aspect is further that the present membrane surfaces aresmooth thus causing less fouling of the surface.

The membranes according to the invention mainly have a symmetrical porestructure in the cross-section as a result of which they may beconsidered as absolute membranes. Besides they are especially suitablefor backwashing.

The hydrophilic polymer is converted into an essentially non-swollenstate prior to cross-linking by removing of the undesirable componentsfrom the polymer solution, like solvents; and from the coagulationmedium like for instance water.

Such components may be removed in an efficient way by means of anon-solvent for the hydrophilic polymer. If one uses as hydrophilicpolymer polyvinylpyrrolidone and as hydrophobic polymer polyetherimideand performs the coagulation in a water medium then in the first placethe obtained membrane is washed with ethanol after removal from thecoagulation medium and subsequently with hexane as a non-solvent for thehydrophilic polymer.

As an alternate the undesirable components may be removed from themembrane, e.g. water or other components, by means of conditioned air orgas or freezedrying after the membrane has been removed from thecoagulation medium.

After removal of the above mentioned undesirable components from themembrane the cross-linking of the essentially dry hydrophilic polymerthat is left behind in or on the polymer matrix, is accomplished byheating the membrane up to a temperature between 50° and 350° C.

The cross-linking of the hydrophilic polymer may also be performed in aproper way by means of suitable reagents e.g. toluenediisocyanatedissolved in hexane. Besides, cross-linking may be obtained byirradiation.

After cross-linking of the hydrophilic polymer a very stable and sturdypolymer matrix with pores is obtained, having an excellent permeabilityfor the permeate.

It is to be understood that in the case of the known hydrophilicmembranes such a stable polymer matrix with an open pore structure isout of the question.

Examples of hydrophobic polymers which, according to the presentprocess, may be successfully applied for the preparation of themembranes according to the invention, are: polysulfone,polyethersulfone, polyetherketone, poly-2,6-dimethyl, 1,4-phenyleneoxydeand derivatives therefrom, polyetherimide, polyamides, polyimides,polyvinylidenefluoride, ideenfluoride etc., however, without anyrestriction.

As hydrophilic polymer a.o. polyacrylic acid, polyvinylalcohol,polyvinylacetate, polyvinylpyrrolidone, polyethyleneglycol,polyvinylpyridine, polyethyleneimine, etc. may be used, without anyrestriction.

According to the invention excellent results are obtained if ashydrophobic polymer polyetherimide and as hydrophilic polymerpolyvinylpyrrolidone is used.

As a solvent--according to the invention--commonly N-methylpyrrolidoneis used although also other suitable solvents may be applied, asdimethylsulfoxide, dimethylformamide, dimethylacetamide, etc.

Very suitable hydrophilic membranes are obtained if the ratio ofpolyetherimide/polyvinylpyrrolidone/N-methylpyrrolidone is 10-35% byweight/5-35% by weight/85-30% by weight and preferably 15-20% byweight/10-15% by weight/75-65% by weight.

It is to be observed that according to the present process flat, as wellas tubular membranes, on a carrier or otherwise, may be prepared asmembranes of a hollow fibre structure.

Furthermore, the invention ranges to hydrophilic microfiltration and/orultrafiltration membranes flat or tubular of a hollow fibre structure,consisting of a hydrophobic polymer and a hydrophilic polymer, whichhydrophilic polymer is cross-linked and is fixed in or on the polymermatrix, the membrane having pores of 0.001-10 μ, a heat-resistance of upto 200° C., a water permeability of up to 8000 l/m² /hr/bar, a goodchemical resistance and a good mechanical sturdiness.

The process according to the invention will now be explained in moredetail by way of the following non-limitative examples.

EXAMPLE I

A solution was prepared of 17 parts, by weight, of polyetherimide (Ultem1000, trademark General Electric), 13 parts, by weight, ofpolyvinylpyrrolidone (M 360,000 Dalton) in 70 parts, by weight, ofN-methylpyrrolidone (NMP). The polymer solution thus obtained was spuninto a hollow fibre, a method which is known per se. The obtained hollowfibres were transferred into a coagulation medium consisting of waterwith a temperature of 20°-70° C.

After removal of the hollow fibres from the coagulation medium thefibres were dewatered by treatment with successively ethanol and hexane.

After this treatment the fibres are practically free of water in whichstate they were subjected to a heat treatment of 150° C. during 10-30hours in order to crosslink the polyvinylpyrrolidone present in or onthe membrane matrix.

The fibres obtained in this manner have a good resistance to chemicals,have a good mechanical sturdiness and have a heat-resistance of up to200° C. and higher.

The fibres have a water permeability of 3000-8000 l/m² /hr/bar and apore size of 0.5-2 μ.

This membrane has a water absorbing capacity of only 4% based on thetotal weight at a relative humidity of 100% and at a temperature of 25°C.

EXAMPLE II

A hollow fibre membrane was prepared according to the process asdescribed in Example I on the understanding that first of all a solutionof 16 parts by weight of polyetherimide and 20 parts by weight ofpolyvinylpyrrolidone (M 40,000 Dalton) was prepared in 64 parts ofweight of N-methylpyrrolidone (NMP). This polymer solution wassubsequently spun into hollow fibre membranes.

The hollow fibres obtained in this manner have, in addition to thefavourable properties mentioned in Example I, a water permeability of200-3000 l/m² /hr/bar at a pore size of 0.05-0.5 μ.

EXAMPLE III

From a polymer solution, prepared according to Example I, flat membraneswere cast. As a coagulation medium water was used with a temperature of20°-70° C., whereas the time between casting and immersing in thecoagulation medium is 10-60 sec. In this manner membranes were obtainedwith pore sizes of 0.01-2 μand a water permeability of 200-2000 l/m²/hr/bar.

EXAMPLE IV

A solution was prepared of 20 parts, by weight, of polyethersulfone(Victrex 200, trademark I.C.I.), 10 parts, by weight, ofpolyvinylpyrrolidone (M 360.000) and 70 parts, by weight, ofN-methylpyrrolidone. The polymer solution obtained in this manner wasspun into a hollow fibre. As a coagulation medium water of 20°-70° C.was used.

After dewatering (ethanol/hexane) the fibre was subjected to a heattreatment during 20 hours at a temperature of 140° C. The waterpermeability of the obtained hollow fibre membranes was 100-3000 l/m²/hr/bar, whereas the size of the pores in the membrane was 0,1-1 μ.

EXAMPLE V

A solution was prepared of 20 parts, by weight, of polyetherimide (Ultem1000, trademark General Electric) and 5 parts, by weight, ofpolyvinylalcohol (M 5000) in 75 parts by weight, of N-methylpyrrolidone.From the thus obtained polymer solution flat membranes were cast,whereby as a coagulation medium water was used at a temperature of20°-70° C. The time between casting and immersing in the coagulationmedium was 10-60 sec. The membrane obtained in this manner was dewateredby means of ethanol/hexane, after which the polyvinylalcohol wascross-linked by means of 1% toluenediisocyanate dissolved in hexane.

This membrane has a pore size of 0.01-0.05 μand a water permeability of10-200 l/m² /hr/bar.

We claim:
 1. Process for the preparation of hydrophilic membrane of apolymer matrix comprisingobtaining a membrane matrix from a solution ofat least one hydrophobic polymer, at least one hydrophilic polymer, anda suitable solvent medium, subjecting the membrane matrix to acoagulation medium, converting the membrane matrix into an essentiallynon-swollen membrane matrix, and cross-linking the hydrophilic polymerwhich is present in or on the essentially non-swollen membrane matrixsuch that the hydrophilic polymer is fixed in or on the polymer matrix.2. Process according to claim 1, characterized in that the membranematrix is converted into an essentially non-swollen membrane matrix bythe removal from the membrane matrix of solvents and undesirablecomponents of the coagulation medium whereby the hydrophilic polymer isconverted into an essentially non-swollen state prior to cross-linking.3. Process according to claim 2, characterized in that the removal ofthe solvents and undesirable components from the membrane matrix takesplace by means of a non-solvent for the hydrophilic polymer.
 4. Processaccording to claim 2, characterized in that the removal of the solventsand undesirable components from the membrane matrix takes place by meansof conditioned air or gas.
 5. Process according to any one of claims 1or 2-4, characterized in that the cross-linking .[.or.]. .Iadd.of.Iaddend.the hydrophilic polymer takes place by heating of the membraneup to a temperature between 50° and 350° C. .Iadd.sufficient tocrosslink.Iaddend..
 6. Process according to any one of claims 2-4,characterized in that the cross-linking of the hydrophilic polymer takesplace by means of reagents.
 7. Process according to claim 2,characterized in that as hydrophobic polymer polyetherimide and ashydrophilic polymer polyvinylpyrrolidone is used.
 8. Process accordingto claim 7, characterized in that as a solvent N-methylpyrrolidone isused.
 9. Process according to claim 8, characterized in that the ratioof polyetherimide/polyvinylpyrrolidone/N-methylpyrrolidone is 10-35% byweight/5-35% by weight/85-30% by weight.
 10. Process according to claim9, characterized in that the ratio ofpolyetherimide/polyvinylpyrrolidone/N-methylpyrrolidone is 15-20% byweight/10-15% by weight/75-65% by weight.
 11. Process according toclaims 2 or 9, characterized in that a flat or tubular membrane isprepared on a carrier or otherwise.
 12. Process according to claims 2 or9, characterized in that a membrane is prepared of a hollow fibrestructure.
 13. Hydrophilic microfiltration- and/or ultra-filtrationmembranes, flat or tubular of a hollow fibre structure consisting.Iadd.essentially .Iaddend.of a hydrophobic polymer and at least onehydrophilic polymer which .Iadd.together form a polymer matrix, said.Iaddend.hydrophilic polymer .[.is.]..Iadd.being .Iaddend.cross-linkedand fixed in or on the polymer matrix, .Iadd.while the polymer matrix isessentially non-swollen .Iaddend.. the membrane having pores of 0.001-10μ, heat-resistance of up to 200° C., a water permeability of up to 8000l/m² /hr/bar, a good chemical resistance and a good mechanicalsturdiness.